In the construction of window assemblies adapted for installation within the side walls of a metal building or other types of buildings, it is common to use extruded aluminum frame sections which are assembled to form a rectangular frame for surrounding one or more rectangular window glazings or glass panels to form a window unit. A weather-tight seal is provided between the window panels and the surrounding rectangular aluminum frame, and usually the assembled unit is installed between horizontally spaced vertical metal channels or studs which interrupt the horizontal wall girts within one or more of the side walls of the metal building. Sometimes, the extruded aluminum sections or members which form the rectangular frame are provided with extruded plastics connecting elements which form thermal barriers to limit heat transfer through the frame.
It has been found that the above described form of window unit or assembly requires substantial time for installation within the wall of the building. The aluminum window frames also have a relatively low "R" insulation value and usually require the preconstruction of horizontally spaced vertical metal studs or channels within the wall where a window unit is to be located. The aluminum frame window units are also relatively expensive in construction, and it is not economically practical to offer the aluminum frames in a variety of different anodized colors.